1. Field of the Invention
The present invention relates to a process for producing carbocyclic nitriles or heterocyclic nitrites (hereinafter commonly referred to as "nitrile compounds") by reacting a carbocyclic or heterocyclic compound having organic substituent(s), with ammonia and an oxygen-containing gas. More particularly, the invention relates to a process in which unreacted ammonia is recovered from the reaction product gas and recycled to the reaction system.
Carbocyclic nitriles are useful as raw materials for the production of synthetic resins, agricultural chemicals or the like, or as intermediate products of amines, isocyanates or the like. Also, heterocyclic nitrites are useful as intermediate products of medicines, feed additives, food additives or the like.
2. Description of the Prior Art
The nitrile compounds have been produced by reacting a hydrocarbon compound with ammonia and a oxygen-containing mixed gas, i.e., by so-called ammoxidation. When carbocyclic or heterocyclic nitrites are synthesized by ammoxidation of corresponding carbocyclic or heterocyclic compounds having organic substituent(s), a large amount of heat is generated as compared to ammoxidation of olefins. Accordingly, the ammoxidation of carbocyclic or heterocyclic compounds is advantageously carried out by vapor-phase fluid catalytic reaction to facilitate removal of the reaction heat and avoid the occurrence of side reactions due to local heating.
As catalysts for the vapor-phase fluid catalytic reaction, there have been proposed various catalyst systems comprising a metal oxide which may or may not be supported on a carrier such as silica and alumina. For example, Japanese Patent Publication No. 49-45860 discloses a process for producing aromatic nitriles by subjecting alkyl-substituted aromatic compounds to ammoxidation. Japanese Patent Application Laid-Open No. 63-190646 discloses a process for the ammoxidation of alkyl-substituted aromatic compounds or alkyl-substituted alicyclic compounds using Fe/Sb-based catalysts.
Also, Japanese Patent Application Laid-Open No. 1-275551 discloses a process for the ammoxidation of alkyl-substituted aromatic compounds or alkyl-substituted alicyclic compounds using V/Cr/B/Mo-based catalysts. Japanese Patent Application Laid-Open No. 5-170724 discloses the similar process using Mo/P-based catalysts. Japanese Patent Application Laid-Open No. 9-71561 discloses a process for the production of dicyanobenzene by ammoxidation of xylene using Fe/Sb/V-based catalysts.
In these ammoxidation reactions, ammonia is used in an excessive amount relative to stoichiometric amount thereof in order to produce respective nitrites corresponding to the alkyl-substituted aromatic compounds and alkyl-substituted alicyclic compounds at a high yield. Usually, the ammonia is used in an amount of 3 to 7 moles per mole of one organic substituent contained in the raw material. In industrial-scale operations, from economical viewpoint, unreacted ammonia is preferably recovered from the reaction gas and recycled to the reaction system.
There have been conventionally proposed various processes in which after separating nitrile compounds from the ammoxidation reaction product gas, unreacted ammonia is recovered from the residual gas and recycled to the reaction system.
In "Monthly Report of Japan Chemical Society", vol. 22, pp. 419-451 (1972), there is disclosed the process for producing isophthalonitrile by the ammoxidation of m-xylene wherein unreacted ammonia is recovered from the reaction product gas after separating the isophthalonitrile therefrom and recycled to the reaction system, and the residual waste gas is converted into harmless state, and discharged.
In addition, "Chemical Process Collection" (edited by Institute of Chemical Engineering; pp. 749-752) discloses the process for the production of benzonitrile. In this process, toluene, ammonia and air are subjected to ammoxidation in the presence of a Mn/W-based catalyst by fixed bed method. The reaction product gas is cooled to 10.degree. C. or lower to collect benzonitrile and water. Then, the residual gas is introduced into an ammonia recovering tower and separated into waste gases (carbon dioxide gas, carbon monoxide, nitrogen, oxygen, etc.) and ammonia. The thus recovered ammonia is recycled to the reaction system. The liquid collected by cooling is separated into a water phase and an organic phase containing benzonitrile, and then the organic phase is introduced into a distillation tower where the organic phase is separated into low boiling substances (such as unreacted toluene, ammonia, hydrogen cyanide, water, etc.) and crude benzonitrile. Further, the crude benzonitrile is introduced into a purification tower, and separated into benzonitrile and high boiling substances.
In the above process, the reaction product gas is cooled to collect the liquid containing benzonitrile, and then only ammonia recovered from the residual gas is recycled to the reaction system. However, in this process, since the reaction product gas is cooled to 10.degree. C. or lower, the collected liquid inevitably contains large amounts of ammonia and ammonium carbonate, thereby failing to recover ammonia contained in the liquid. "Hydrocarbon Processing", pp. 103-106, (February, 1976) discloses the process for producing aromatic nitrites by conducting the ammoxidation in the presence of a metal oxide catalyst by vapor-phase fluid catalytic reaction. In this process, after the reaction product gas is cooled in a cooler to collect the nitrile compounds therefrom, the residual gas is introduced into an absorption tower where unreacted ammonia and by-produced hydrogen cyanide are dissolved in water to separate these compounds from waste gases (carbon monoxide, nitrogen, etc.). The thus obtained aqueous solution is then introduced into a diffusion tower and separated into waste liquid containing high boiling substances and a fraction containing ammonia, hydrogen cyanide, water or the like. The fraction is further introduced into a distillation tower and separated into ammonia and an aqueous solution (containing hydrogen cyanide and ammonium carbonate). Then, the aqueous solution is separated into a mixture of hydrogen cyanide and a carbon dioxide gas, and ammonia-containing water. The ammonia-containing water is returned to the preceding distillation tower. The nitrile compounds collected by cooling is separately purified.
Thus, in the above process, after separating the nitrile compounds from the reaction product gas, only ammonia recovered from the residual gas is recycled to the reaction system. For this reason, this process is more advantageous as compared to the previous benzonitrile production process because ammonia contained in ammonium carbonate can also be recovered. However, in this process, the use of additional facilities such as diffusion tower, distillation tower and ammonium carbonate-decomposition tower is required to separate ammonia and hydrogen cyanide (containing carbon dioxide) from the residual gas, resulting in increased costs for facilities.
In "Chemical Engineering", pp. 53-55 (November, 1971), there is disclosed the process for the production of isophthalonitrile. In this process, m-xylene, ammonia and air are subjected to ammoxidation using a vanadium oxide catalyst by fixed bed reaction. The reaction product gas is introduced into a scrubber and water-cooled therein to crystallize isophthalonitrile. The residual gas is further introduced into an absorption tower where unreacted ammonia and by-produced hydrogen cyanide are dissolved in water, and the waste gas discharged from the top of the tower is burned for disposal. In the above document, it is described to recover ammonia from the aqueous solutions produced in the scrubber and absorption tower and recycled the recovered ammonia to the reaction system. However, no concrete method therefor is specified therein. The isophthalonitrile-containing water slurry condensed in the scrubber is introduced into a filter to separate isophthalonitrile therefrom, and the separated isophthalonitrile is then dehydrated and dried to obtain the aimed product.
When carbocyclic or heterocyclic compounds having organic substituent(s) are reacted with ammonia and an oxygen-containing gas such as air, the reaction product gas contains unreacted carbocyclic or heterocyclic compounds, nitrile compounds, ammonia, hydrogen cyanide, a carbon dioxide gas, water, carbon monoxide, nitrogen, oxygen and the like. Usually, the reaction product gas contains the by-produced hydrogen cyanide at a concentration of about 1%. The by-produced hydrogen cyanide is readily converted into formamide, polymers or the like due to heat history, and the thus produced compounds are contained in waste water, thereby increasing a total oxygen demand (TOD) of the waste water and causing coloration thereof. Also, when the reaction product gas is cooled, carbon dioxide contained in the gas is readily precipitated as ammonium carbonate. Therefore, in the above process for the production of carbocyclic or heterocyclic nitrites, when nitrile compounds are removed from the reaction product gas and then unreacted ammonia is recovered from the residual gas, it is important not only to avoid operational troubles of apparatuses due to the conversion of hydrogen cyanide and precipitation of ammonium carbonate, but also to reduce the burden for treating the waste water and increase a recovery rate of unreacted ammonia.
Thus, in the conventional processes described in the above prior arts, after separating nitrile compounds from the reaction product gas, only ammonia recovered from the residual gas is recycled to the reaction system. Therefore, the recovery rate of ammonia is low. The by-produced hydrogen cyanide is burned, or otherwise converted into polymers or the like and contained in the waste water. In addition, in order to separate ammonia from hydrogen cyanide, it is required to use additional facilities such as distillation tower and ammonium carbonate decomposition tower, resulting in increased installation costs for facilities.
The present inventors have studied the recovery of ammonia upon the production of isophthalonitrile by ammoxidation of m-xylene. As a result, it has been found that when fresh ammonia is used as a raw material, the aimed nitrile compounds is produced at a high yield and the activity of catalyst used is prevented from being deteriorated with time, but when ammonia recovered by the above processes is used, the ammoxidation reaction is inhibited, so that the amount of nitrile compounds as the aimed product is reduced, and the activity of the catalyst is considerably deteriorated due to promoted sintering, thereby failing to stably obtain the aimed product for a long period of time.